Conventionally, in an image forming apparatus based on the digital system, for calibrating output characteristics of an output device such as a printer (image forming unit) and also for emphasizing an area having particular density, there has been used an image signal conversion table (Look-Up Table and described as LUT hereinafter).
An image forming apparatus generally comprises an image reading unit, an image processing unit, an image writing unit, and image output unit, and the LUT described above is incorporated in the image processing unit and converts an input image signal inputted from the image reading unit to the image processing unit and outputs the converted image signal as an output image signal to the image writing unit.
This LUT utilizes the output characteristics such as density of an image outputted from a printer, so that, when output characteristics of the printer changes due to "degradation or contamination" of the image forming unit or other related sections, the LUT can not play a role for calibration.
To overcome this problem, as a control generally called as process control executed in an image forming apparatus, sometimes a plurality of patterns each having a different image density are formed on an image carrier such as a light-sensing body or a transfer member, a reflected light from or a transmitting light through the patterns is detected, and such parameters as a charged potential, a development bias, or a exposure rate of a laser are changed or the gradation calibration table is changed according to a result of the detection above.
The calibrating method described above provides the merit that calibration is automatically executed in an image forming apparatus without requiring manual labor of an operator, but because of the characteristics of an optical sensor, there is no sensibility in the high density side where a deposition rate of toner is high, and calibration is made only in a range from a low density area to an intermediate density area where a toner deposition rate is rather low. Also with the calibrating method above, fluctuation in a quantity of transferred toner caused by gradually change of the transfer performance of a transfer section and fluctuation of image density due to change in the fixing capability in a fixing section can not be corrected.
In contrast, there is a method in which a pattern image formed on an image carrier and transferred and fixed on a transfer member is read with a scanner, and the gradation calibration table is selected or prepared according to the read data or a color conversion coefficient/RGB-YMCK color conversion table is prepared according to the read data. In this method, different from the calibrating method using an optical sensor described above, treatment by an operator such as manually placing a discharged transfer member on a document base or the like is required, but calibration of a high image density section where a toner deposition rate is high is possible, and also change of image density due to gradually change in performance of the transfer section as well as to change in the fixing capability in the fixing section can advantageously be corrected.
The invention disclosed in Japanese Patent Laid-Open Publication No. HEI 7-264411 relates to an image forming apparatus comprising a pattern forming unit for forming a gradation pattern on a recording member, a reading unit for reading the gradation pattern formed by the pattern forming unit, and a regulating unit for regulating conditions for image formation according to the gradation pattern read by the pattern reading unit, in which more gradation pattern steps are provided in a density area where the gradation characteristics is not linear as compared to those in other density areas. In this invention, the gradation patterns are fixed.
By the way, to execute regulation of image density, sometimes a gradation pattern is outputted onto transfer paper, the gradation pattern is read with a scanner, and the conversion characteristics of a gradation calibration table in an image processing section is decided according to the read gradation pattern. This operation is called automatic color calibration (ACC). In this step, to simplify processing executed no software and also to reduce a quantity of toner used for preparing a gradation pattern, it is necessary to precisely decide a gradation calibration table with a small number of gradation patterns.
When the number of gradation patterns is small, if a write value for gradation patterns is fixed, sometimes precision in regulation may not be stabilized due to effects by the development characteristics gradually changing. Description is made below for the reason with reference to FIG. 31.
In FIG. 31, a horizontal axis of the first quadrant indicates a laser write value for a gradation pattern, while a vertical axis indicates a gradation pattern read value outputted onto transfer paper, which indicates a relation between a gradation pattern write value and a scanner read value, and a horizontal axis of the second quadrant indicates a toner deposition rate on a light-sensing body, and this value indicates a relation between a toner deposition rate on the light-sensing body and a scanner read value. A vertical axis of the third quadrant indicates a development potential, which indicates a development characteristics of a printer. The development potential indicates a difference between a surface potential on the light-sensing body and a DC component of development bias, and the larger the value is, the higher a deposition rate of toner onto the light-sensing body is. The fourth quadrant indicates a relation between a development potential and a gradation pattern write value.
The reference numerals n (1) and n (2) on the horizontal axis of the first quadrant indicates write values in the first and second stages of a gradation pattern respectively, and herein a write value for a stage 0 of the gradation pattern is 0, which indicates a read value for background of the transfer paper.
The reference numerals g1) and h1) in the first quadrant show cases each having different development characteristics, and g1) indicates a case where a toner deposition rate onto a light-sensing body is large, and the case is indicated by the phrase of "Development rate is large" in the figure. The reference numeral h1) indicates a case where a toner deposition rate onto the light-sensing body is standard, and the case is indicated by the phrase of "Development rate is standard". Herein it is assumed that scanner read values to gradation pattern write values n [1], n [2] for the development characteristics of g1), h1) are Ag [n[1]], Ag [n[2]], Ah [n[1]], and Ah [n[2]]. As shown by the graph, Ag [n[1]] and Ah [n[1]] are substantially the same values. Assuming that the value is A [1], a difference value A [1]-Ag [n[2]] and A [1]-Ah [n[2]] for the read values in the first stage and second stage of the gradation pattern for the development characteristics g1), h1) correspond to (e) .DELTA.Ag [1] and (f) .DELTA.Ah [1] in the figure respectively. As understood from the figure, .DELTA.Ag [1] is larger than .DELTA.Ah [1], so that sometimes the error becomes larger when a gradation calibration table is prepared by estimating a laser write value between n [1] and n [2] by means of linear interpolation or spline interpolation.